The present invention relates to a resistor for use in a high density wiring circuit.
A chip resistor disclosed in the Japanese Patent Laid-open No. 3-62901 may be cited as an example representing the conventional resistors. In the following, the conventional resistor is described in the order of manufacturing process steps with reference to FIG. 8 which shows the cross sectional structure.
A pair of base electrodes 3 are formed at both ends of an insulating ceramic chip substrate 2; by printing a pattern with a gold resin paste which contains gold as the metal organic material, using a known photo etching process or a screen printing process, and baking it in an approximate temperature of 850.degree. C. The thickness of the base electrode 3 film formed is thin, because the metal organic material used here contains more organic components relative to metal components. The use of the above paste leads to an advantage of saving in gold consumption, and therefore a reduction of manufacturing cost.
A resistor film 4 is formed by printing a pattern with a Ru group paste with each of the respective ends overlapped to the pair of base electrodes 3, using a known screen printing process and baking it in an approximate temperature of 850.degree. C. As the base electrodes 3 have been formed with a conductive material containing gold and glass frit a gold system material, diffusion of silver to the resistor film 4, which is observed when the electrode is made of a silver group material, does not occur. Therefore, there is no deterioration in the electrical characteristics of the resistor film 4.
In order to alleviate possible influence of trimming operation, to be made in a subsequent process step, on the resistor film 4, a glass undercoat is applied in the form of an undercoat film 5 on the resistor film 4. The resistor film 4 can be a thin film resistor of Ni--Cr system alloy, Ni--Cr--Al system alloy, Ni--Cr--Fe system alloy, etc. formed through a deposition or other such process. In this case, the undercoat film 5 is not provided.
Then, an Ag resin paste 6, which being an electro-conductive resin material having a thermo-setting temperature of between 150-250.degree. C. and a strong adhesive property to an inorganic material, is applied to cover the entire surface of the base electrodes 3, and baked. If in this case an electro-conductive film of Ag or Ag/Pd is formed on the base electrodes 3 by a high-temperature baking process of approximately 850.degree. C., the electrical characteristics may be affected at the boundary region. This is a why such an electro-conductive resin material of low baking temperature is used. Film thickness of the base electrode 3 is as thin as several hundred .ANG.. The covering with the Ag resin paste 6 improves electrical contact between the base electrode 3 and a measuring groove during the trimming operation. The covering is useful also to cover up a weakness of the gold-containing base electrode 3 since it easily wears.
The resistance is adjusted to a certain specific value by a known trimming method. An overcoat film 8 is formed over the resistor film 4 in order to protect it from adverse environments during the forthcoming plating process and in the actual operating conditions. An end face electrode 7 is formed at both ends of the chip substrate 2. The face electrode 7 is then plated to form a plating film 9, and a finished chip resistor is completed.
In the above described conventional resistor, however, the film thickness after baking of the base electrode 3 is very thin because it uses a metal resin paste to avoid the deterioration of resistor film 4 in electrical characteristics, and the adhesion to substrate 2 is weak. Therefore, if a resistor undergoes a thermal amplitude for a certain time period the base electrode 3 itself may have cracks causing a substantial shift in the resistance value, or in a worst case the electrical continuity is broken.
Furthermore, because the electric conduction between the end face electrode 7 and the resistor film 4 is made via the Ag resin paste 6, when a resistor is used in a sulfidizing ambient, a chlorinating ambient or in other strong corrosive environments, the film of Ag resin paste may be corroded creating a broken electric conduction with the resistor film 4, or a substantial shift in the resistance value. The Ag resin paste 6 is normally covered by the plating film 9 so as not to be exposed to the outside in the actual operating conditions, a stress due to the thermal history of soldering often causes a gap between the plating film 9 and the overcoat film 8 rendering the Ag resin paste 6 exposed to the outer surface. The Ag resin paste 6 may then be corroded.